When someone mentions the term language aptitude, even laypeople think they understand the concept that lies behind it: they tend to assume, probably rightly, that some people have a greater talent for learning foreign languages than others. However, the technical definition of foreign language aptitude is more restricted and more detailed. On the one hand, it does not imply that some people can learn foreign languages while others are incapable of it; it only concerns the rate of learning, that is, progress made over a given period of time, but not ultimate attainment (Carroll, 1973; Ehrman, & Oxford, 1995). On the other hand, aptitude is not hypothesised to be a unitary construct, but rather a cluster of different cognitive traits that are advantageous as far as foreign language learning is concerned (Carroll, 1981; Grigorenko, Sternberg, & Ehrman, 2000; Robinson, 2012; Skehan, 1998). The exact nature and the relative importance of these factors thus depends to a great extent on the theory of language aptitude proposed by different authors (Dörnyei & Ryan, 2015). The lack of a generally accepted, theoretically motivated, and empirically testable definition, which is a fundamental problem of the area of language aptitude research, is well reflected by the fact that since the birth of commercial aptitude batteries “language aptitude is what language aptitude tests measure” (Dörnyei, 2005, p. 35.).